The invention relates to improvements in light assemblies for aiding a lift truck operator in positioning a load-handling device with respect to a load to be picked up, or with respect to the place where the load is to be deposited. More specifically, the assembly is useful in aiding the operator in accurately positioning a load-handling device vertically, particularly when the device is elevated above the operator. The assembly also provide a second function of providing general illumination for traveling or load handling when the load-handling device is in a lowered position.
In the past, lights have been mounted either atop a lift truck overhead guard or upon a lift truck load carriage to provide general illumination to aid a lift truck operator during traveling and during the manipulation of loads in dimly lit areas. While lights on the overhead guard provide sufficient illumination for lower positions of the load carriage, they are normally blocked by the load at intermediate elevations of the carriage and are ineffective when the carriage is in a high-lift position where the operator must be able to determine the relative elevation between the position of the load forks and the pallet cavities or other spaces into which the forks must be inserted. Even if the direction of such lights is controllable such that the light may be directed upwardly toward a high-lift position, or if illumination at the high-lift position is otherwise good due to effective ambient lighting, the operator still encounters great difficulty in accurately positioning the load forks vertically such that, when the lift truck is advanced toward a stack for engaging a particular load, the forks are at a proper elevation so as to slip into the pallet cavity or other space beneath a load. This is because the operator's line of sight with respect to the fork tips is at a steep upward angle making it difficult for him to gauge level relationships at a high elevation from a vantage point below. A similar problem exists with respect to depositing a load at an elevated position on a stack or storage rack.
Conversely, lights which are mounted on a lift truck carriage normally must be positioned at an elevation considerably above the forks to avoid damage to the lights from contact with the floor or other obstacles when the forks are in a lowered position. The necessity of elevating a carriage-mounted light above the forks results in the light being blocked by the load in many instances and therefore rendered ineffective.
Allen et al U.S. Pat. No. 3,122,957 discusses, in the text thereof, various prior types of devices utilized to indicate shelf height in those applications where storage racks are utilized, and in particular is directed to a photosensor assembly mounted on the load carriage which directs light against the storage racks and coordinates with reflectors mounted at predetermined positions on the storage rack to reflect the light back to a carriage-mounted photosensor which acts through a potentiometer to indicate to the lift truck operator the height of the forks relative to the rack-mounted reflectors. Aside from the complexity of such system, its usefulness is limited by the fact that is can be used only in a storage rack application where reflectors are mounted at predetermined locations on the rack. It is not versatile enough to be used, for example, in many conventional warehouse situations where there are no storage racks and loads are merely stacked atop one another separated by spacers defining openings into which the forks may be inserted and withdrawn.